The thickness of silicon-carbide (SiC) substrates in SiC-based power semiconductor devices can affect the forward voltage needed to operate the devices at a given current level. In particular, the performance and/or operation of SiC devices such as SiC Schottky diodes, MOSFETs, BJTs, PiN diodes, n-channel IGBTs, thyristors and/or vertical JFETs may be affected by the relatively high resistance of thick SiC substrates. For example, n-type, 4H-SiC substrates may account for about 1 mohm-cm2 of the specific on-resistance of various devices. This may constitute about 50% of the on-resistance of a 600 V SiC Schottky diode, and/or about 90% of the on-resistance of a 300V SiC Schottky diode. A p-type 4H-SiC substrate may add about 50-100 mohm-cm2 to the device on-resistance. For this reason, it has not been practical to develop vertical devices, such as GTOs and n-channel IGBTs, on p-type SiC substrates.
Present SiC device fabrication technology typically employs relatively thick (300-400 microns) substrates. Fabrication processing, including the backside ohmic contact anneal, may be performed on epilayers grown on the substrate. Since there may be subsequent processing steps after ohmic contact formation, the substrate is typically thick enough to provide adequate mechanical support for the epilayers. However, the thick substrate used for mechanical support may add to the electrical and/or thermal resistance of the device.
Ohmic contacts may be formed on SiC substrates at low/room temperatures by, for example, implanting ions into a surface of a SiC wafer on which the contact is to be formed. In some conventional approaches, the ohmic contacts may be formed by implantation of dopants into a backside of a SiC wafer. However, if an implanted doped SiC substrate is thinned prior to formation of ohmic contacts, the doped region may be removed during the thinning, which may make the implant superfluous. Accordingly, metals deposited for ultimately forming ohmic contacts may not have ohmic properties when deposited on the substrate, as the implant may be performed in a later step. Ion implantation for the formation of ohmic contacts is discussed, for example, in U.S. patent application Ser. No. 09/787,189, and in U.S. Patent Publication No. 2002/0179910, the disclosures of which are incorporated herein by reference in their entireties.